1. Field of the Invention
The present invention relates to an electric spot welding gun. In particular, the present invention relates to an electric spot welding gun in which one electrode tip among a pair of electrode tips used in spot welding is opened and closed relative to the other electrode tip using a servo motor.
2. Related Art
Conventionally, in this type of electric spot welding gun, rotational motion of the rotor of the servo motor is converted to advancing or retracting motion in an axial direction through a feed screw mechanism configured by a ball screw and a nut. An opening/closing operation of one electrode tip is performed using this advancing and retracting motion (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-028788).
A schematic block diagram of a conventional electric spot welding gun 110 is shown in FIG. 1. As shown in FIG. 1, the electric spot welding gun 110 includes a motor drive unit 130 including a servo motor and a feed screw mechanism, and a push rod 150 that is made to advance or retract by way of the rotation of the servo motor in the motor drive unit 130. One electrode tip 171 is mounted to an arm 170 fixed to a leading end of the push rod 150.
In addition, with the electric spot welding gun 110, the timing at which current is flowed between both electrode tips when the one electrode tip 171 moves to a certain position is precisely determined; therefore, the movement position of the one electrode tip 171 relative to the other electrode tip (not illustrate) must be precisely known.
As a result, an encoder 160 for detecting the rotation angle of the rotor of the servo motor has conventionally been provided.
With the conventional electric spot welding gun 110, the encoder 160 is connected in series to the rear of the rotor (opposite side to the feed screw mechanism side thereof extending from the rotor forward) of the servo motor. In other words, the rotating shaft of the encoder 160 is disposed coaxially at a position at which the rotating shaft of the rotor of the servo motor extends rearwards.
In this case, the rotating shaft of the encoder 160 is mounted to the rear of the rotating shaft of the rotor either directly or using a joint such as a coupling, whereby the rotation of the rotor is transmitted to the encoder 160.
In addition, for the purpose of not restricting the shapes of the components to be the welding targets, the electric spot welding gun 110 in which the press point (position of the electrode tip 171) is offset from the push rod 150 (pressing axis by the servo motor), as shown in FIG. 1, has been used recently. With such an electric spot welding gun 110, the moment acts on the push rod 150 during pressing.
As a result, with the conventional electric spot welding gun 110, a structure is established in which the shaft bushing 200 is disposed at a forward portion of the motor drive unit 130, and the moment acting on the push rod 150 during pressing is received by this shaft bushing 200.